Directional control valve

ABSTRACT

A pressure compensated spool type directional control valve characterized in that it has a compensating piston which senses the pressure drop across a metering orifice whose flow area is determined by the position of the spool in the valve housing and which is operative to divert excess flow to the reservoir when the pressure drop across the metering orifice exceeds a predetermined value, said compensating piston being so operative in conjunction with the operation of any one of the spools of a plural spool directional control valve. The compensating piston may also be operated as an unloading valve to bypass pump output to the reservoir when the spool or spools of the directional control valve are in neutral position, or as a relief valve by employment of a pilot relief valve member in association with the side of the compensating piston which is exposed to pressure downstream of the metering orifice. Another characterizing feature of the pressure compensated directional control valve herein is that each spool has a secondary compensating piston therein which decreases the pressure drop flow forces at the spool metering orifice. In a two-spool directional control valve the compensating piston adjusts to the lower load demand when both spools are simultaneously shifted to an operating position. In one form of two-spool directional control valve having a pressure compensating piston as aforesaid, check valves are provided to make adjustment to the higher load when both spools are simultaneously shifted to an operating position.

United States Patent [191 Malott et a1.

[ 1 Aug. 6, 1974 DIRECTIONAL CONTROL VALVE [75] Inventors: Thomas .1. Malott, Mentor; John C.

Paul, Richmond Heights, both of Ohio [73] Assignee: Parker-Hannifin Corporation,

Cleveland, Ohio [22] Filed: May 5, 1972 [2]] Appl. No.: 250,793

[52] U.S. Cl 137/117, 91/414, 91/446, 91/451, 137/596.l2 [51] Int. Cl. Fl5b 11/02 [58] Field of Search. 137/106, 117, 596.12, 596.13; 91/414, 451; 60/468 Primary Examiner-Robert G. Nilson Attorney, Agent, or FirmDonnelly, Maky, Renner & Otto [57] ABSTRACT. A pressure compensated spool type directional control valve characterized in that it has a compensating piston which senses the pressure drop across a metering orifice whose flow area is determined by the position of the spool in the valve housing and which is operative to divert excess flow to the reservoir when the pressure drop across the metering orifice exceeds a predetermined value, said compensating piston being so operative in conjunction with the operation of any one of the spools of a plural spool directional control valve. The compensating piston may also be operated as an unloading valve to bypass pump output to the reservoir when the spool or spools of the directional control valve are in neutral position, or as a relief valve by employment of a pilot relief valve member in association with the side of the compensating piston which is exposed to pressure downstream of the me tering orifice.

Another characterizing feature of the pressure compensated directional control valve herein is that each spool has a secondary compensating piston therein which decreases the pressure drop flow forces at the spool metering orifice. In a two-spool directional control valve the compensating piston adjusts to the lower load demand when both spools are simultaneously shifted to an operating position. In one form of two-spool directional control valve having a pressure compensating piston as aforesaid, check valves are provided to make adjustment to the higher load when both spools are simultaneously shifted to an operating position.

12 Claims, 4 Drawing Figures DIRECTIONAL CONTROL VALVE BACKGROUND OF THE INVENTION Pressure compensated flow control valves per se are known in the art and generally comprise a compensating piston which is exposed to pressure upstream and downstream of a metering orifice in the valve, the movement of the compensating piston responsive to a pressure drop across the metering orifice greater than desired being effective to maintain a constant desired flow through the metering orifice. Such pressure compensated flow control valves may be any of several types, namely, the restrictive flow type wherein the movement of the compensating piston progressively blocks off the outlet flow area thus restricting or throttling flow through the valve; the bypass flow type in which the compensating piston movement diverts or bypasses excess flow through the metering orifice to the reservoir; and the combination bypass and restrictive type in which the movement of the compensating piston not only bypasses excess flow but also restricts the output flow from the valve.

The known pressure compensated flow control valves may of course be employed in various circuits such as a meter-in closed center circuit wherein the pressure compensated flow control valve is installed between the pump and the directional control valve or a meter-out closed center circuit wherein the pressure compensated flow control valve is installed between the motor and the reservoir. In an open-center circuit, the pressure compensated flow control valve may be installed in meter-in or meter-out relation between the directional control valve and the motor.

SUMMARY OF THE INVENTION The present invention relates to a pressure compensated spool-type directional control valve in which a compensating piston in the valve housing senses the pressure drop across a variable area orifice which is determined by the position of the spool in the valve housing, the compensating piston being movable under the influence of a pressure drop exceeding a predetermined value to divert excess flow to the reservoir. The compensating piston also constitutes an unloading valve to divert the pump output flow to the reservoir when the spool is in a neutral position blocking communication between the inlet and motor ports of the directional control valve.

The directional control valve herein is further characterized in that a single compensating piston may constitute a flow control valve for a plurality of spools contained in the directional control valve housing.

The directional control valve herein is further characterized in that in addition to the compensating piston aforesaid, each spool has associated therewith a secondary compensating piston and check valve, the secondary compensating pistons providing for lower pressure drop flow forces at the spool metering forces and, when a plurality of spools are simultaneously actuated, the check valves associated with the secondary compensating pistons make adjustment to the higher load, i.e., the

secondary compensating piston associated with the spool handling the lower load demand restricts the flow The directional control valve herein is still further characterized in that the compensating piston operates as a main relief valve member when a pilot relief valve member on one side thereof which is exposed to pressure downstream of the variable area orifice vents the pressure acting on said one side of the compensating piston more rapidly than downstream fluid pressure may flow through a restricted orifice to said one side of said compensating piston, thus to bypass fluid pressure in excess of a predetermined maximum for which the pilot relief valve member is set.

The directional control valve herein is still further characterized in that when it has two or more spools therein, the compensating piston senses the pressure drop across the variable area orifices defined by the settings of the respective spools when simultaneously actuated through check valves whereby the highest downstream pressure is sensed by the compensating piston, i.e., the least pressure drop or smallest variable area orifice of the simultaneously actuated spools. Each spool also has a secondary compensating piston and a check valve associated therewith, the downstream pressures of the secondary compensating pistons being equalized when the spools are simultaneously actuated so that the pressures downstream of the check valves will throttle the flow to the motor having the lowest load pressure.

The directional control valve herein is further characterized in the provision of a flow control valve of the pressure compensated type aforesaid which, in addition, has an auxiliary compensating metering-out valve between the motor and the directional control valve to control return flow from the motor to the reservoir via a variable area orifice defined by the setting of the spool, the compensating piston of the auxiliary unit sensing pressure upstream and downstream of said variable area return orifice to progressively throttle the return flow.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-section view of one form of pressure compensated directional control valve embodying the present invention;

FIG. 2 is a cross-section view of another form of pressure compensated directional control valve;

FIG. 3 is a cross-section view of yet another form of pressure compensated directional control valve; and

FIG. 4 is a fragmentary cross-section view illustrating a pressure compensated directional control valve which embodies a pressure compensating piston disposed in meter-out relation between a fluid motor and the spool of the directional control valve.

DETAILED DESCRIPTION OF THE INVENTION The pressure compensated directional control valve 1 illustrated in FIG. 1 is, for example, of the closed center four-way type which has a housing 2 with a bore 3 in which the spool 4 is reciprocable, said bore 3 being intersected axially therealong starting at the middle by the center branch 5 of a pressure feed passage, a pair of inlet passages 6 straddling said feed passage branch 5, a pair of feed passage branches 7 straddling said inlet passages 6, a pair of motor passages 8 straddling said feed passages 7, and a pair of return passages 9 mad dling said motor passages 8. The middle branch 5 of the feed passage has a load check valve 10 therein through which fluid under pressureflows to the feed pressure branches 7 when the spool 4 is shifted in the bore 3. In the neutral position of the spool as shown in FIG. 1, the center land 11 blocks communication between the inlet passages 6 and the center branch 5 of the feed passage, the lands 12 block communication between the inlet passages 6 and the adjacent feed passage branches 7, and the lands l4 block communication between the motor passages 8 and the adjacent feed passages 7 and return passages 9.

The housing 2 has inlet, motor, and return ports 15, 16, and 17 respectively communicated with a pump P, a fluid motor 18 and a tank or reservoir 19 as shown. The spool 4 is held in the neutral position by means of a conventional spring centering mechanism 20 and when the spool 4 is shifted to the right from neutral position, fluid under pressure is conducted to the motor 18 via the left-hand metering slots or variable area orifices 21, the load check valve 10, the right feed passage 7 and the right motor passage 8, and return fluid from the motor 18 is conducted to the tank 19 via the left motor passage 8 and left return passage 9. Conversely, when the spool 4 is shifted to the left from neutral position fluid under pressure is conducted to the motor 18 via the right-hand set of variable area orifices 21, the load check valve 10, the left feed passage 7 and left motor passage 8, and return fluid from the motor 18 is conducted to the tank 19 via the right motor passage 8 and right return passage 9.

The rate of flow of fluid to the motor 18 is controlled by the position of the spool 4 in the housing 2 which varies the flow areas of the respective sets of metering slots 21.

To provide for pressure compensation, the housing 2 has therein a pressure compensating piston 23 which senses the pressure drop across either set of variable area orifices 21, the upstream pressure being sensed at one end of the compensating piston 23 through the passage 24 and the downstream pressure being sensed through the passage 25 which leads from feed passage 5 to a chamber 26 at the other end of the compensating piston 23. A spring 27 biases the compensating piston 23 to the postion shown whereat communication is blocked between the inlet port and the return passage branch 28. The downstream pressure chamber 26 is communicated with the return passage 28 via a tiny orifice 29 which prevents buildup of high pressure in said chamber 26 when the spool 4 is in neutral position, as might occur due to slight leakage of fluid past the land 11 into the center feed passage branch 5. When the pressure drop across either set of variable area orifices 21 exceeds a predetermined value to indicate a flow rate exceeding a predetermined amount for which the spool 4 has been set, the pressure in the downstream chamber 26 will decrease relative to the pressure in the upstream chamber 30 thus to progressively urge the compensating piston 23 toward the left against the biasing spring 27 to divert excess flow to the return passage 28, thereby to maintain an approximately constant flow to the motor 18 irrespective of load pressure.

As evident, when the valve 1 shown in FIG. 1 has another one or more spools 4, the single compensating piston 23 will control the flow to any one of the respective motors 18 simply by providing an additional passage 25 or passages 25 leading to the respective center feed passage branches 5. In cases where plural spools 4 may be simultaneously actuated, reference may be had to FIGS. 2 and 3, for example, to obtain compensation for different load demands of the simultaneously actuated motors associated with the respective spools.

When the spool 4 of FIG. 1 is in neutral position, the downstream chamber 26 pressure is substantially at return passage 28 pressure via the tiny orifice 29 and hence the pump P output pressure in acting on the upstream chamber will shift the compensating piston 23 to a bypassing position whereby the pump P output will be diverted to the tank 19. The compensating piston 23 thus operates as an unloading valve when the spool 4 is in neutral position blocking flow of pressure fluid to the feed passages 5 and 7. However, movement of the spool 4 to operating position will at once effect a rise in pressure in the downstream pressure chamber 26 to cause movement of the compensating piston 23 to an appropriate position depending on the pressure drop across either the left-hand or right-hand set of variable area orifices 21.

The pressure compensated directional control valve illustrated in FIG. 2 comprises a housing 36 having three bores 37, 38, and 39 in which the spools 40, 41 and compensating piston 42 respectively are reciprocable, said bores 37, 38 and 39 being intersected by an inlet passage 43 and by a return passage 45 which has branches 46 and 47 additionally intersecting the spool bores 37 and 38. The spool bores 37 and 38 are additionally intersected by motor passages 48 and 49 which are connected to the respective fluid motors 50 and 51 and by a sensing passage 52 which is communicated by way of the orifice 53 with the downstream pressure chamber 54 at one end of the compensating piston 42, the latter having communication with the upstream pressure via the orifice 56 which communicates with the chamber 57 at the other end of the compensating piston 42. From the sensing passage 52 there is another orifice 58 which communicates the sensing passage 52 with the return passage 47.

Each spool 40 and 41 has two sets of variable area orifices 59 for setting the desired flow area according to the position of the spool 40 or 41 in its spool bore 37 or 38 with reference to the inlet passage 43. Each spool 40 and 41 also contains therein a secondary compensating piston 60 which as it is moved in the spool 40 or 41 defines a secondary variable area orifice with the spool openings 61 and when the spool 40 or 41 is in an operating position, the pressure downstream of both variable area orifices 59 and 61 unseats the check valve 62 in the secondary compensating piston 60 and passes through the set of openings 63 communicating with the sensing passage 52 into the downstream pressure chamber 54 either to hold the compensating piston 42 in the position shown or permitting movement thereof toward the right as viewed in FIG. 2 to bypass or divert a portion of the pump P output from the inlet passage 43 to the return passage 45 via the openings 64 in the compensating piston 42 depending on the magnitude of the pressure drop sensed by the compensating piston 42 being less than or greater than the predetermined pressure drop across the spool variable area orifice 59 and secondary variable area orifice 61.

When both spools are in neutral position as shown in FIG. 2, the pressure in the downstream chamber 54 of the compensating piston 42 is essentially at return pressure via the orifices 53 and 58 and hence pressure in the upstream chamber 57 will urge the compensating pass the pump P output from the inlet passage 43 to the return passage 45 and tank 67 thus to constitute an unloading valve. When either spool 40 or 41 is moved to the right or to the left, the variable area orifice 59 will be set according to the position of the spool in its bore to establish a predetermined rate of flow to the associated motor 50 or 51 to move the secondary compensating piston 60 to permit such flow and the downstream pressure opens the check valve 62 and flows through the spool passages 63 into the sensing passage 52 and into the downstream chamber 54 to move the compensating piston toward the left from its unloading position to decrease the bypass flow therethrough if the pressure drop across the variable area orifice 59 is less than desirable. As the pressure drop across the variable area orifice 59 increases to a point where the flow rate is greater than for which the spool 40 or 41 is set, the pressures sensed by the compensating piston 42 will cause movement of the compensating piston 42 toward the right to divert excess flow from the inlet passage 43 to the return passage 45.

In the event that both spools 40 and 41 are simultaneously shifted to operating position setting the fiow rates as desired, the sensing passage 52 will be effective to equalize the pressures in the secondary chambers 68 at the highest load pressure which also will be present in the downstream pressure chamber 54. The secondary compensating pistons 60 are effective to decrease pressure drop flow forces at the spool variable area orifice 59, and the check valves 62 make adjustment to the highest load when the spools 40 and 41 are simultaneously actuated to operating position.

A further characterizing feature of the FIG. 2 valve is that the downstream sensing chamber 54 has associated therewith a pilot relief valve member 69 to cause the compensating piston 42 to operate as a main relief valve member in case the pressure in the downstream chamber 54 exceeds a predetermined maximum value. When the pressure exceeds such predetermined value, the pilot relief valve member 69 will open the chamber 54 to the return passage 45 thus to vent the chamber 54 more rapidly than fluid can be replenished thereinto via the orifice 53 and hence the reduction of pressure in the chamber 54 permits the higher pressure in the upstream pressure chamber 57 to urge the compensating piston 42 to the right to relieve excess pressure from the inlet passage 43 into the return passage 45.

The pressure compensated directional control valve 75 shown in FIG. 3 is generally similar to that shown in FIG. 2 except for the general arrangement of passages and therefore the same reference numerals except with prime symbols have been used to denote corresponding parts and passages. In FIG. 3 the passage 76 equalizes the pressures in the secondary chambers 68 through openings 63 in the spools 40 and 41 f when both spools are in operating position. In the event that both spools 40 and 41 are simultaneously shifted to operating position setting the flow rates as desired, the passage 76 and openings 63 will be effective to equalize the pressures in the secondary chambers 68 at the highest load pressure. The secondary compensating pistons 60 'are effective to decrease pressure drop flow forces at the spool variable area orifices 59', and the check valves 62 make adjustment to the highest load when the spools 40 and 41 are simultaneously actuated to operating position.

In FIG. 3 the spools 40 and 41 have check valves 78 which permit flow from the downstream sides of the spool variable area orifices 59 into the downstream sensing chamber 54' of the compensating piston 42 thereby maintaining in the chamber 54 through orifice 53' a pressure corresponding to the highest load pressure if both spools 40 and 41' are simultaneously actuated to operating position. Again, as in FIG. 2, each spool 40 and 41 has therein a secondary compensating piston 60' and check valve 62' and there is provided an equalizing passage 76 as aforesaid which intercommunicates the downstream chambers 68 of said secondary compensating pistons 60' with each other. The FIG. 3 valve also has a pilot relief valve member 69' operative in a manner similar to that described in connection with FIG. 2 whereby the main compensating piston 42 will function as a relief valve in the event that the pressure in the downstream chamber 54 exceeds a predetermined value for which the pilot relief valve member 69 is set. In the directional control valve illustrated in FIG. 4 there is secured on the directional control valve housing 86, a meter-out pressure compensating valve 87 which comprises a compensating piston 89 which senses the pressure drop across the orifice 90 defined between the motor passage 91 and the edge 92 of land 93 of the spool 94 whereby when the pressure drop across such variable area orifice 90 exceeds a predetermined value, the compensating piston 89 is moved toward the right to progressively throttle the return flow from the motor 95 to the return passage 96 in the directional control valve housing 86 through the passages 97. The pressures upstream and downstream of the orifice 90 are sensed in the chambers 98 and 99 at opposite ends of the compensating piston 89, the upstream pressure in chamber 98 being sensed through opening 88 in the compensating piston 89.

When the spool 94 is shifted to the right the flow of pressure fluid to the motor 95 will be via the inlet passage 106, the load check valve 100 in the spool, the spool groove 101, motor passage 91, and check valve 102 to bypass the auxiliary compensating piston 89. Return flow from motor 95 thence passes through motor passage 103 and spool groove 104 to the return passage 105.

The orifices 29 (FIG. 1), 58 (FIG. 2), and 58' (FIG. 3) have such small flow capacities as to be substantially saturated when the respective spools are shifted to operating positions. When the respective spools are in neutral position, the orifices 29, 58, and 58 prevent buildup of pressure in the chambers 26, 54, and 54 due to minute leakage over the spool lands.

We, therefore, particularly point out and distinctly claim as our invention:

1. A pressure compensated spool type directional control valve comprising a housing having at least two bores each intersected axially therealong by inlet, motor, and return passages adapted for connection to a fluid pressure source, to respective fluid motors, and a tank respectively; a spool reciprocable in each bore from a neutral position blocking communication of its motor passage with said inlet and return passages to respective first and second operating positions selectively communicating its motor passage with said inlet passage or said return passage, each spool and said housing defining a metering orifice having a variable flow area dependent upon the extent of movement of said spool from neutral position toward said first operating position; and a single pressure compensated flow control compensating piston movable in another bore in said housing and having one end portion exposed to fluid pressure upstream of said orifices and having the opposite end portion exposed to fluid pressure downstream of the respective orifices; said compensating piston being moved by excessive pressure differential upstream and downstream of said metering orifices to divert excess flow from said inlet passage to said return passage hence to maintain a preselected rate of flow of fluid through said valve irrespective of which one of said spools is moved toward its said first operating position; the flow path to the motor passage having the lower load pressure during simultaneous actuation of said spools to said first operating position having a secondary compensating piston defining a secondary variable area orifice through which fiuid from its metering orifice flows to said motor passage; said secondary compensating piston being exposed to the higher load pressure in the other motor passage and being moved thereby to decrease the flow capacity of said secondary variable area orifice.

2. The valve of claim 1 wherein check valve means in said housing opens to expose said secondary compensating piston to the pressure downstream of said secondary variable area orifice in the absence of such higher load pressure and closes to prevent flow of such higher load pressure fluid to the motor passage having the lower load pressure.

3. The valve of claim 2 wherein said secondary compensating piston is movably disposed in that spool which controls the flow of fluid to the motor passage having the lower load pressure; and wherein said check valve means is movable in said secondary compensating piston.

4. The valve of claim 1 wherein said secondary com-' pensating piston is movably disposed in that spool which controls the flow of fluid to the motor passage having the lower load pressure.

5. A pressure compensated spool type directional control valve comprising a housing having at least two bores each intersected axially therealong by inlet, motor, and return passages adapted for connection to a fluid pressure source, to respective fluid motors, and a tank respectively; a spool reciprocable in each bore from a neutral position blocking communication of its motor passage with said inlet and return passages to respective first and second operating positions selectively communicating its motor passage with said inlet passage or said return passage, each spool and said housing defining a metering orifice having a variable flow area dependent upon the extent of movement of said spool from neutral position toward said first operating position; and a single pressure compensated flow control compensating piston movable in another bore in said housing and having one end portion exposed to fluid pressure upstream of said orifices and having the opposite end portion exposed to fluid pressure downstream of the respective orifices; said compensating piston being moved by excessive pressure differential upstream and downstream of said metering orifices to divert excess flow from said inlet passage to said return passage hence to maintain a preselected rate of flow of fluid through said valve irrespective of which one of said spools is moved toward its said first operating position; each flow path to the respective motor passages having a secondary compensating piston defining a secondary variable areaorifice through which fluid from its metering orifice flows to its motor passage when the associated spool is in said first operating position; each secondary compensating piston being exposed to pressure downstream of its secondary variable area orifice via check valve means .in said housing; said housing having passage means to equalize the pressures to which said secondary compensating pistons are exposed when said spools are simultaneously actuated to said first operating position; said check valve means preventing reverse flow of fluid to the motor passage of lower load pressure whereby the higher load pressure to which the associated lower load pressure secondary compensating piston is exposed via said passage means is effective to decrease the flow capacity of its secondary variable area orifice.

6. The valve of claim 5 wherein said other end portion of said single compensating piston is exposed to pressure in said passage means downstream of said secondary variable area orifices.

7. The valve of claim 5 wherein said other end portion of said single compensating piston is exposed to pressure in additional passage means communicating via other check valve means with portions of the flow paths downstream and upstream of the respective metering orifices and secondary variable area orifices.

8. The valve of claim 7 wherein said other check valve means opens for exposure of said other end portion, upon actuation of either spool to said first operating position, to the fluid pressure of said portion of the flow path; said other check valve means, upon simultaneous actuation of said spools to said first operating position, closing communication with said portion of the flow path leading to the motor passage of lower load pressure.

9. The valve of claim 5 wherein said spools in neutral positions, substantially close the respective metering orifices for increase of the pressure differential acting on said one and other end portions of said single compensating piston whereby the latter is moved to divert fluid under pressure in said inlet passage to said return passage thus to constitute an unloading valve to unload the pressure source adapted to be connected to said inlet passage.

10. The valve of claim 5 wherein said housing has relief valve means responsive to fluid pressure acting on said other end portion exceeding a predetermined maximum to relieve such excessive pressure to said return passage whereby predominant fluid pressure acting on said one end portion moves said single compensating piston to relieve suchpredominant fluid pressure from said inlet passage to said return passage.

11. The valve of claim 10 wherein said housing has orifice means to decrease the fluid pressure acting on said other end portion when said relief valve means is open to said return passage.

12. The valve of claim 5 wherein said valve has bleed orifice means communicating the fluid pressure acting on said other end portion with said return passage to decrease such downstream pressure when said spools are in neutral position. 

1. A pressure compensated spool type directional control valve comprising a housing having at least two bores each intersected axially therealong by inlet, motor, and return passages adapted for connection to a fluid pressure source, to respective fluid motors, and a tank respectively; a spool reciprocable in each bore from a neutral position blocking communication of its motor passage with said inlet and return passages to respective first and second operating positions selectively communicating its motor passage with said inlet passage or said return passage, each spool and said housing defining a metering orifice having a variable flow area dependent upon the extent of movement of said spool from neutral position toward said first operating position; and a single pressure compensated flow control compensating piston movable in another bore in said housing and having one end portion exposed to fluid pressure upstream of said orifices and having the opposite end portion exposed to fluid pressure downstream of the respective orifices; said compensating piston being moved by excessive pressure differential upstream and downstream of said metering orifices to divert excess flow from said inlet passage to said return passage hence to maintain a preselected rate of flow of fluid through said valve irrespective of which one of said spools is moved toward its said first operating position; the flow path to the motor passage having the lower load pressure during simultaneous actuation of said spools to said first operating position having a secondary compensating piston defining a secondary variable area orifice through which fluid from its metering orifice flows to said motor passage; said secondary compensating piston being exposed to the higher load pressure in the other motor passage and being moved thereby to decrease the flow capacity of said secondary variable area orifice.
 2. The valve of claim 1 wherein check valve means in said housing opens to expose said secondary compensating piston to the pressure downstream of said secondary variable area orifice in the absence of such higher load pressure and closes to prevent flow of such higher load pressure fluid to the motor passage having the lower load pressure.
 3. The valve of claim 2 wherein said secondary compensating piston is movably disposed in that spool which controls the flow of fluid to the motor passage having the lower load pressure; and wherein said check valve means is movable in said secondary compensating piston.
 4. The valve of claim 1 wherein said secondary compensating piston is movably disposed in that spool which controls the flow of fluid to the motor passage having the lower load pressure.
 5. A pressure compensated spool type directional control valve comprising a housing having at least two bores each intersected axially therealong by inlet, motor, and return passages adapted for connection to a fluid pressure source, to respective fluid motors, and a tank respectively; a spool reciprocable in each bore from a neutral position blocking communication of its motor passage with said inlet and return passages to respective first and second operating positions selectively communicating its motor passage with said inlet passage or said return passage, each spool and said housing defining a metering orifice having a variable flow area dependent upon the extent of movement of said spool from neutral position toward said first operating position; and a single pressure compensated flow control compensating piston movable in another bore in said housing and having one end portion exposed to fluid pressure upstream of said orifices and having the opposite end portion exposed to fluid pressure downstream of the respective orifices; said compensating piston being moved by excessive pressure differential upstream and downstream of said metering orifices to divert excess flow from said inlet passage to said return passage hence to maintain a preselected rate of flow of fluid through said valve irrespective of which one of said spools is moved toward its said first operating position; each flow path to the respective motor passages having a secondary compensating piston defining a secondary variable area orifice through which fluid from its metering orifice flows to its motor passage when the associated spool is in said first operating position; each secondary compensating piston being exposed to pressure downstream of its secondary variable area orifice via check valve means in said housing; said housing having passage means to equalize the pressures to which said secondary compensating pistons are exposed when said spools are simultaneously actuated to said first operating position; said check valve means preventing reverse flow of fluid to the motor passage of lower load pressure whereby the higher load pressure to which the associated lower load pressure secondary compensating piston is exposed via said passage means is effective to decrease the flow capacity of its secondary variable area orifice.
 6. The valve of claim 5 wherein said other end portion of said single compensating piston is exposed to pressure in said passage means downstream of said secondary variable area orifices.
 7. The valve of claim 5 wherein said other end portion of said single compensating piston is exposed to pressure in additional passage means communicating via other check valve means with portions of the flow paths downstream and upstream of the respective metering orifices and secondary variable area orifices.
 8. The valve of claim 7 wherein said other check valve means opens for exposure of said other end portion, upon actuation of either spool to said first operating position, to the fluid pressure of said portion of the flow path; said other check valve means, upon simultaneous actuation of said spools to said first operating position, closing communication with said portion of the flow path leading to the motor passage of lower load pressure.
 9. The valve of claim 5 wherein said spools in neutral positions, substantially close the respective metering orifices for increase of the pressure differential acting on said one and other end portions of said single compensating piston whereby the latter is moved to divert fluid under pressure in said inlet passage to said return passage thus to constitute an unloading valve to unload the pressure source adapted to be connected to said inlet passage.
 10. The valve of claim 5 wherein said housing has relief valve means responsive to fluid pressure acting on said other end portion exceeding a predetermined maximum to relieve such excessive pressure to said return passage whereby predominant fluid pressure acting on said one end portion moves said single compensating piston to relieve such predominant fluid pressure from said inlet passage to said return passage.
 11. The valve of claim 10 wherein said housing has orifice means to decrease the fluid pressure acting on said other end portion when said relief valve means is open to said return passage.
 12. The valve of claim 5 wherein said valve has bleed orifice means communicating the fluid pressure acting on said other end portion with said return passage to decrease such downstream pressure when said spools are in neutral position. 